FIG. 9 is a structural view of the conventional optical information recording/reproducing apparatus described on Pages 58 through 62--"LD Array Head For DRAW"--of "Micro-Optics News" (Vol. 3, No. 1, edited on Feb. 4, 1985) by Ito and Ohta.
Referring to FIG. 9, reference numeral 1 designates a semiconductor laser array emitting two light beams of recording beam L1 of high intensity of light and reproducing beam L2 of low intensity of light, 2 designates a collimator lens for collimating the beams L1 and L2 in the parallel light beams emitted from the semiconductor laser array 1, and 3 designates a beam shaping prism for correcting distribution of elliptical strength of the respective collimated beams L1 and L2.
Reference numeral 4 designates a polarizing beam splitter, which is adapted to transmit therethrough the beams L1 and L2 having passed a beam shaping prism, toward an information recording medium (to be discussed below) and to reflect toward an error detecting system (to be discussed below) a recording reflected beam L1' and a reproducing reflected beam L2' from the information recording medium.
Reference numeral 5 designates a reflecting mirror, and 6 designates a 1/4 wavelength plate, which are disposed on an optical path at the transmission side of the polarizing beam splitter 4.
Reference numeral 7 designates an objective lens for focusing on the information recording medium the beams L1 and L2 having passed the reflecting mirror 5 and 1/4 wavelength plate 6 respectively, 8 designates an information recording medium comprising an optical disc rotatable around a rotary shaft 8a, and 9 designates an information track formed concentrically or spirally in the information recording medium 8.
Reference numeral 10 designates a convex lens disposed on an optical path at the reflection side of the polarizing beam splitter 4, and 11 designates a spatial filter disposed at the focal point of convex lens 10, which cuts off the recording reflected beam L1' reflected from the information recording medium 8 and allows only the reproducing reflected beam L2' to transmit through the lens 11.
Reference numeral 12 designates a beam splitter for dividing the reproducing reflected beam L2' having passed the spatial filter 11, toward a tracking error detection system and a focusing error detection system (both to be discussed below), 13 designates a two-divided photodetector for receiving the reproducing reflected beam L2' having passed the beam splitter 12, 14 designates a convex lens, 15 designates a knife edge, and 16 designates a two-divided photodetector for receiving the reproducing reflected beam L2' through the knife edge 15, which are disposed on the optical path at the reflection side of the beam splitter 12.
Reference 17 designates a differential amplifier which takes in a difference between the two signals output from the two-divided photodetector 13, 18 designates a tracking actuator for driving the objective lens 7 in the traversing direction (in the direction of the arrow T) with respect to the information track 9 on the basis of an output signal of the differential amplifier 17, 19 designates a differential amplifier which takes in a difference between the two signals output from the two-divided photodetector 16, and 20 designates a focusing actuator for driving the objective lens 7 in the vertical direction (in the direction of the arrow F) with respect to the surface of the information recording medium 8 on the basis of the output signal of the differential amplifier 19.
In addition, the two-divided photodetector 13 and differential amplifier 17 constitute the tracking error detection system for detecting a tracking error of the reproducing beam L2 irradiated to the information recording medium 8. The convex lens 14, knife edge 15, two-divided photodetector 16 and differential amplifier 19, constitute the focusing error detection system for detecting a focusing error of the reproducing beam L2.
FIG. 10 is an illustration showing the irradiation states of the respective light beams L1 and L2 with respect to the information track 9 in FIG. 9, in which reference numerals P1 and P2 designate two light spots, in other words, a recording spot and a reproducing spot, formed of the condensed beams L1 and L2, the arrow D designates the traveling direction by rotation of the information recording medium 8, and 21 designates a pit formed on the information track 9 by the recording spot P1.
Next, explanation will be given on operation of the conventional optical information recording/reproducing apparatus shown in FIGS. 9 and 10.
The recording beam L1 and reproducing beam L2 emitted from the semiconductor laser array 1 are collimated by the collimator lens 2 to the parallel light beams and further formed by the beam shaping prism 3 into two light beams having nearly symmetrical intensity-distribution with respect to the optic axis rotationally.
Next, the recording beam L1 and reproducing beam L2 are incident on the objective lens 7 through the polarizing beam splitter 4, reflection mirror 5 and 1/4 wavelength plate 6 and focused on the information track 9 at the information recording medium 8 to be the recording spot P1 and reproducing spot P2.
The recording spot P1 in advance with respect to the rotation direction of the information recording medium 8 forms in order on the information track 9 the pits 21 modulated corresponding to the contents of information, the lagging reproducing spot P2 reproducing at the same time the content of information included in the recorded pit.
Continuously, the recording reflected beam L1' and reproducing reflected beam L2' reflected from the information recording medium 8 are again incident on the polarizing beam splitter 4 through the objective lens 7, 1/4 wavelength plate 6 and reflection mirror 5. The reflected beams L1' and L2', which reciprocate through the 1/4 wavelength plate 6 so as to rotate in the polarizing direction of 90.degree., are reflected at the polarizing beam splitter 4 and image-formed on the spatial filter 11 by the convex lens 10. In this case, only the reproducing reflected beam L2' passes through the spatial filter 11, is divided by the beam splitter 12, and received by the two-divided photodetectors 13 and 16.
Accordingly, the tracking error signal is detected by the push-pull method using the two-divided photodetector 13, the focusing error signal being detected by the knife-edge method using the knife edge 15 and the two-divided photodetector 16. The tracking error signal and focusing error signal thus obtained are amplified by the differential amplifiers 17 and 19 so as to drive the tracking actuator 18 and focusing actuator 20 respectively.
Also, the sum of output signals (not shown) of two-divided photodetector 13 is gained to detect the quantity of light of reproducing reflected beam L2' so as to reproduce the information signal recorded on the information track 9 at the information recording medium 8.
The conventional optical information recording/reproducing apparatus records and reproduces the information as the above-mentioned, in which it is generally known that when the push-pull method detects the tracking error signal, a tracking offset becomes larger.
FIG. 11 is a characteristic graph showing the relation between the follow-up quantity of the objective lens 7 with respect to the information track 9 and the tracking offset quantity, which is described in, for example, "Optical head for write-once disk with two perpendicular axes" of "Optical Memory Symposium" (in 1985, Pages 97 through 102), and which shows that, when a track follow-up amount of the objective lens 7 is 100 .mu.m, the tracking offset is generated by only about 0.08 .mu.m. Usually, a tolerance of tracking offset is about 0.05 to 0.1 .mu.m so that it is seen that an offset value of 0.08 .mu.m is about the limit of the tolerance.
FIG. 12 is a characteristic graph showing the relation between the inclination of the information recording medium 8 and the tracking offset quantity, which is described in Pages 224 through 229 of, for example, "On-land Composite Pregroove Method for High Track Density Recording" (by Y. Tsunoda et al), SPIE, Vol. 695, 1986. In this case, it is shown that the information recording medium 8 inclines at an angle of 1.degree. to generate a tracking offset of 0.11 .mu.m to exceed the aforesaid tolerance.
Now, when the radial wobbling method, instead of the push-pull method, is used for the tracking control, it is well-known that the aforesaid tracking offset problem is almost solved, the radial wobbling method including a spot-wobbling method of vibrating the reproducing light spot P2 and a track wobbling method of providing the wobble pit at the information track 9, which are quite equivalent in principle.
The spot-wobbling method has been described in the "Principles of Optical Disc System" [by G. Bouwhuis et al, Adam Hilger Ltd., 1985, Pages 73 through 75]. Also, it is reported by, for example, the "Composite Wobbled Tracking in the Optical Disc System" (by Ohtake et al, Pages 181 through 188, Optical Memory Symposium '85, 1985) that the tracking control by such radial wobbling method is hard to be affected by the inclination or the like of the information recording medium 8.
However, it is difficult to adopt the spot-wobbling method with respect to the conventional apparatus which emits the recording beam L1 and reproducing beam L2 from one light source 1 as shown in FIG. 9. Even when the track wobbling method is adopted, the wobble pits are periodically provided, whereby the tracking state of the light spot cannot always be detected.
Furthermore, the relative-positional relationship of the recording spot P1 is mechanically adjusted to coincide with the position of the reproducing spot P2, but in the conventional apparatus in FIG. 9, in spite of that one objective lens 7 focuses the recording beam L1 and reproducing beam L2 on the information recording medium 8, the focusing error signal and tracking error signal are obtained only from the reflected light of the reproducing spot P2. Accordingly, when the information recording medium 8 becomes eccentric to even slightly deteriorate the parallelism between the line connecting two light spots P1 and P2 and the information track 9, the tracking offset is generated at the recording spot P1.
The conventional optical information recording/reproducing apparatus, as above-mentioned, detects the tracking error signal by use of the push-pull method, thereby creating the problem in that the tracking offset generated by the track follow-up of objective lens 7 or the inclination of information recording medium 8 becomes larger. Also, since the focusing error signal and tracking error signal are obtained only from the reflected light of the reproducing spot P2, the information recording medium 8 is eccentric to deteriorate the parallelism between the line connecting the light spots P1 and P2 and the information track 9, thereby creating the problem in that the tracking offset is generated to make impossible the stable tracking control.